The Human Microbiome Project (HMP) consortium established a unique population-scale framework which characterized the relationship between the human host and its microbial communities. These data provide strong initial evidence for host influences on microbial community structure and underscores the capacity for metagenomics and metabolomics to explore host-pathogen interactions in disease states. We will adopt such a systems biology approach to extend our published and preliminary findings as they relate to Clostridium difficile infection (CDI), antibiotic resistance and treatment outcome i children and adults. By comparisons with our pediatric and adult HMP reference datasets, we provide much needed insight into how antibiotics affect intestinal ecosystems over multiple life stages. Because young children are generally asymptomatic carriers of C. difficile whereas adults often become symptomatically infected, our proposed studies provide a developmental perspective of intestinal ecosystems that modulate C. difficile virulence and drug resistance. The novelty of our work is our discovery of an intestinal ecosystem that is refractory to frontline antibiotic therapy, the result of which is treatment failure in CDI patients. Our project goal is t identify microbes that regulate host susceptibility to C. difficile through characterization of newy-identified molecular and biochemical pathways. The combination of cutting edge multi-omics, coupled with real-time measurement of antibiotic resistance and clinical phenotyping in patients is expected to generate a valuable resource that provides new discovery into host susceptibility to CDI. To achieve these objectives we will pursue two aims: Aim 1: Unbiased longitudinal multi-omic studies of host-microbe interactions in CDI development. Aim 2: Targeted mechanistic studies of host-microbe interactions that affect treatment outcome in CDI. Through these longitudinal multi-omics studies, we expect to define host-microbe interactions that are predictive of antibiotic treatment failure in CDI patients and provide a rich array of resources - supported in part by our own ongoing CDI patient registry, the Texas Department of State Health Services, Autism Speaks, the TMC Digestive Disease Center and integrated microbiome centers (Center for Metagenomics and Microbiome Research (CCMR) and Texas Children's Microbiome Center (TCMC)).